SomaFM.com rocks.

My favourite source of online streaming music is, and has been for quite a while now, somafm.com

Last time I checked, they offered 7 different "channels":

Groove Salad
"A tasty plate of ambient beats and grooves. Takes the edge off work."

Secret Agent
"The soundtrack for your stylish, mysterious, dangerous life. For Spies and P.I.'s too!"

Drone Zone
"Served best chilled, safe with most medications. Droning atmospheric textures with minimal beats."

indie pop rocks
"New and classic indie tracks."

cliqhop idm
"Blips'n'beeps backed mostly w/beats."

Beat Blender
"A late night blend of deep-house and downtempo chill."

Boot Liquor
"American Roots music for saddle-weary drunkards."

My favourite is Secret Agent but everybody has different tastes, so get over to somafm.com where you'll find a range of bitrates as well as playlists and how you can help support them. They deserve it!

You'll need to have a suitable player such as WinAmp (Windows), iTunes (Mac) or XMMS (Unix)

If none of those take your fancy, have a look at SHOUTCast or iM Radio